If you send an encrypted email, then the person who's receiving it should also use the same service. Along with the email you will also send the decryption key and when it reaches the destination, that service which operates the email and decryption knows how to decrypt it. It's a thing similar to SSL (Secure Sockets Layer). The email is decrypted when it reaches the destination. If everything is OK, the decryption is done "under the hood" and the person sees the email as a normal email.